This invention relates to hard-of-hearing aid appliances, and more particularly to an improvement of a hearing aid appliance in which an induction loop, in which signal currents corresponding to voices flow, is magnetically coupled to the sensitive reception coils of hearing aids.
The hearing aid appliance according to the invention is intended to be used for teaching or training a number of hard-of-hearing children by suitable placement of induction loops in the classroom, for instance.
It is well known in the art that the employment of a hearing aid appliance with an induction loop is superior to the employment of individual hearing aids the microphones of which are used to receive voice signals, in that the S/N ratio is excellent and difficulties or troubles in hearing are reduced which otherwise may be caused when hard-of-hearing children are far away from the instructor or by ambient noises. Nowadays, the use of a hearing aid appliance is essential in teaching or training hard-of-hearing children. However, hearing aid appliances of this type should satisfy at least the following conditions, because a plurality of hearing aid appliances are often used simultaneously in a plurality of classrooms adjacent to one another (including classrooms built one on another, such as classrooms on the first and second floors), and a number of hard-of-hearing children will move in the classroom.
(1) In the case where a plurality of hearing aid appliances are used in a plurality of classrooms adjacent to one another, the difference in level between the magnetic field strengths in adjacent classrooms should be of a value such that no signal interference occurs.
(2) The magnetic field strength in a classroom should not be greatly varied in the hearing area and in a plane at a height from the floor which corresponds to the height of the sensitive reception coils of the hearing aids held by the hard-of-hearing children.
A variety of hard-of-hearing aid appliances have been proposed in the art; however, none of them satisfy the above-described conditions. The basic arrangement of the induction loop of a conventional appliance is such that only one induction loop is laid so as to surround the ceiling or the floor of the classroom. The distribution of magnetic field strength in a central sectional area of the classroom with the induction loop thus laid is as shown in FIG. 1. The difference between the magnetic field strength at a height of 0.5 m above the floor and that at a height of 2.5 m above the floor is about 6 dB at the center. It can be readily understood from this that signals in adjacent classrooms (especially in the classrooms which are laid one on another) interfere with one another.
In order to minimize the signal interference, a method has been proposed in which a plurality of induction loops having shorter sides are laid. However, this method is not practical in use because the magnetic field is greatly varied (or dips) in the vicinity of the induction loops.